This invention relates to a method of accessing for retrieving a desired track from a number of tracks formed on a rotary recording medium and more particularly to an access method for positioning a light spot at the desired track by using a coarse actuator and a fine actuator, and to an information memory apparatus utilizing the access method.
An optical disc type memory apparatus has been developed as an information memory apparatus which can record/reproduce or erase as necessary, in a contactless manner, informatin on a rotary recording medium of high recording density. In such an optical disc type memory apparatus, a coarse actuator such as a linear motor cooperates with a fine actuator such as a galvanomirror to position a light beam (see Japanese Patent Unexamined Publication NO. 58-91536 corresponding to U.S. pat. application Ser. No. 443,399 filed Nov. 22, 1982 and Continuation Application therefrom Ser. No. 736,125 filed May 20, 1985), now U.S. Pat. No. 4,607,358 (Maeda et. al.). In an access operation with this optical type memory apparatus, a recording/reproducing optical head is first positioned to an approximate destination by using only the coarse actuator and thereafter, a light beam is positioned to a destined track by means of the fine actuator carried on the optical head. However, the fine actuator is dynamically freed during access operations of the coarse actuator and under high speed access operations of the coarse actuator, the fine actuator is subjected to so high an acceleration that it is deflected, with the result that at the termination of an access operation of the coarse actuator, the fine actuator continues to vibrate around a destination. This leads to a problem that a considerable amount of time is required for the fine actuator to position the light beam in the succeeding phase or the fine actuator fails to effect positioning. To solve this problem, it is conceivable to lock the fine actuator in a neutral position during the access operation of the coarse actuator by using a neutral detector operative to detect the neutral position of the fine actuator.
In order to confine within one track a vibration of the light spot due to a deflection of the fine actuator, a vibration angle .theta. of the mirror must be ##EQU1## where f represents a focal distance of an object and p a track pitch, indicating that the vibration angle is required to be a very small angle which is approximately .theta.&lt;0.01 degree for practical apparatus. Locking of the mirror with this accuracy requires that the S/N ratio of a lock servo system be sufficiently large, resulting in necessity of a high gain neutral position detector capable of converting into a large signal an extremely slight angular displacement of the mirror from its neutral position. With this high gain neutral detector, such factors as temperature dependent change in the quantity of light from a light emitting element, a temperature dependent change in the dark current of a photodetector and a change in the reflection factor of the mirror due to aging cause the neutral position detection signal to DC drift to thereby prevent the mirror from being locked with high accuracies.
Moreover, a neutral position detector immune from such variations would be of a complicated structure